When you log in, click network settings. At the bottom it will list all the current DHCP clients. Find the PS3 in the list. Then up above that section there is a part where you can reserve an ip for a specific mac address. Click the drop down box and it will list all the current systems connected to the network. Choose the PS3 from the list and click the '<<' button. Then click save settings. From now on, the PS3 will always have the same ip address (probably 192.168.0.101 or something). Now you can use that ip address for the port forwarding.